The invention relates to a current loop of the 4 mA -20 mA or of the 0-20 mA type, connecting an analog sensor to an acquisition system respectively carrying a senor current and an acquisition current.
Such current loops are in widespread use. The 4 mA -20 mA type loop, e.g. made of xe2x80x9c2 xcexcIsxe2x80x9d technology enables the sensor to operate using the energy supplied by a 4 mA sensor current. The advantages of a current loop are well known: firstly the power supply for the sensor is carried by the same wires as the signal, thereby reducing the cost of cabling compared with other types of signal that would require additional wires in the cable, and secondly the signal is disturbed to a very small extent by electromagnetic radiation, thereby enabling it to be conveyed over long distances or through surroundings having high radiation density.
In known manner, proper operation of the acquisition system is monitored by means of test devices designed to simulate the operation of the sensor. Simulation is performed by connecting the test device so that it takes the place of the analog sensor. Nevertheless, there are drawbacks in disconnecting the sensor: there is a risk of it being wrongly reconnected, e.g. by reversing its polarity, or even that reconnection will be forgotten, or perhaps that the connections will be left too loose. Under such conditions, maintenance of the acquisition system turns out to be counterproductive.
Also in known manner, the operation of the analog sensor is monitored by disconnecting the current loop. This is done, in particular, when the sensor is removed from its installation site. In that case also, disconnection is not without its drawbacks: as a general rule, the acquisition system interprets the open loop as being anomalous and generates an alarm. It is therefore necessary to take action to prevent the anomaly being treated as such by a unit that controls the acquisition system.
The object of the invention is to remedy the problem of monitoring the operation of an acquisition system or of a sensor by disconnection and reconnection in a current loop of the 4 mA-20 mA type or of the 0-20 mA type.
The invention is based on the idea of inspecting the current loop without opening it.
To this end, the invention provides a current loop of the 4 mA-20 mA type or of the 0-20 mA type, connecting an analog sensor to an acquisition system respectively carrying a sensor current and an acquisition current, the loop being characterized in that a test circuit is connected in parallel with the current loop to inject a superposition current into said loop, which current is superposed on the sensor current or the acquisition current.
The superposition current injected into the current loop by the test circuit is superposed on the current carried by the sensor to simulate its operation relative to the acquisition system, or it is superposed on the current passing through the acquisition system to simulate its operation relative to the analog sensor.
The test circuit connected in parallel with the current loop thus serves to inject a superposition current without opening the current loop connecting the acquisition system to the analog sensor. This remedies the drawbacks mentioned above: firstly the risk of the sensor being reconnected with reverse polarity is eliminated, and secondly no open loop anomaly is detected by the acquisition while the analog sensor is being tested.
According to a first advantage of the invention, the test circuit comprises a variable voltage generator connected in parallel with the acquisition system to inject the superposition current by adding it to the acquisition current, thereby making it possible to monitor a low-current threshold of the acquisition system.
In a preferred embodiment, the test circuit includes an ammeter connected in series with the variable DC voltage generator to determine the magnitude of the superposition current.
In another preferred embodiment, the test circuit includes a diode connected in series with the variable voltage generator to protect the current loop when the variable voltage is zero.
In another preferred embodiment, the test circuit includes a diode connected in series with the acquisition system to preserve operating independence of a plurality of current loops connecting a plurality of sensors to a common acquisition system.
According to a second advantage of the invention, the test circuit includes a variable current regulator connected in parallel with the analog sensor to inject the superposition current by being added to the sensor current, thereby making it possible to monitor a high-current threshold of the acquisition system.
According to a third advantage of the invention, the test current includes a variable current regulator connected in parallel with the analog sensor to inject the superposition current by adding it to the sensor current, the superposition current being servo-controlled to said sensor current, thereby making it possible to maintain the acquisition current in the current loop.
In a preferred embodiment, the test circuit includes an ammeter connected in series with the variable current regulator to determine the magnitude of the simulation current.